This invention relates to a motor vehicle hub for a vehicle having an anti-lock braking system and in particular to a cast iron hub having an integrally formed exciter ring.
Today, anti-lock braking systems, commonly known as ABS, are being found on an increasing number of motor vehicles. The ABS detects relative wheel rotation to prevent an impending wheel lock-up condition from occurring. Lock-up of the vehicle wheels is undesirable since it can result in both directional instability and increased vehicle stopping distances.
Advanced ABS designs employ a sensor at each wheel subject to ABS control. The sensors detect individual wheel rotation using various technologies, including variable reluctance sensors and inductive pickups. Signals from the sensors are fed into an ABS computer which determines whether wheel lock-up is impending and which, in response thereto, modulates the braking forces being applied to the vehicle wheels to prevent an impending wheel lock-up condition from occurring.
The variable reluctance and inductive pickup sensors are mounted to a stationary portion of the vehicle wheel assembly and use some type of tone wheel or exciter ring that rotates with the wheel. Hereinafter, the term "exciter ring" will be used to refer to any such member, the rotation of which is detected by the sensors. One common type of exciter ring is a cylindrical ring, formed by a strip of steel (or other material), that is perforated with a number of axially aligned slots. The strip is bent into the shape of a ring, with its two ends butt welded together, and is press fit over a cylindrical surface of the wheel hub so that it will rotate with the hub. The sensor is mounted to a non-rotating component of the vehicle's suspension and is spaced apart from the exciter ring.
Several problems are associated with such an exciter ring. First, it is necessary to maintain a precise clearance between the sensor and the exciter ring. This clearance is on the order of 0.030-0.050 inches. This small controlled clearance between the sensor and the exciter ring produces a higher sensor output to the ABS control resulting in better low speed response in the ABS system. If this clearance is not precisely maintained, the sensor, and hence the ABS, will not function as intended. With a press fit exciter ring fitted onto the hub, tolerance stack-ups can result in radial run out and variability in the clearance between the exciter ring and the sensor. Another problem is that the hoop stresses, formed in the exciter ring as a result of being press fit onto the hub, can cause failure of the butt weld and a resulting loss of the exciter ring. A further problem is created by corrosion of the exciter ring and/or the hub. Corrosion can also result in separation of the exciter ring from the hub. The first problem can result in improper functioning of the ABS while the second and third problems can result in a total ABS failure.
Accordingly, a principal objective of the present invention is to provide a hub and exciter ring assembly that overcomes the above limitations.
The present invention overcomes these limitations by unitarily or integrally forming the exciter ring as a part of the hub itself, rather than forming the exciter ring out of a separate component which must be mounted to the hub. One way to integrally forming the hub and exciter ring is to bond a steel ring to the hub by insert casting the ring in the hub. After casting, the hub is machined to expose the ring at the surface of the hub. The hub and ring are then machined to produce the necessary clearance between the hub surface and sensor. Grooves are subsequently machined into the ring to form spaced teeth around the hub.
To reduce the costs associated with insert casting, the present invention unitarily forms the exciter ring with the hub by machining the exciter ring directly into the hub surface. The hub is initially cast without having teeth for the exciter ring formed in it. Gaging off of bearing cups mounted to the interior cylindrical surface of the hub, the exterior cylindrical surface of the hub is machined to provide the proper clearance between the hub surface and the ABS sensor. Again gaging off of the bearing cups, grooves are cut into the exterior cylindrical surface of the hub forming spaced apart teeth. No additional machining is required.
By gaging off of the bearing cups during machining of the teeth and the exterior cylindrical surface, precise tolerances can be maintained for positioning the sensor relative to the exterior cylindrical surface. By gaging off of the bearing cups, surface roundness of the hub surface in general and the exciter ring portion in particular are also ensured.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.